Opt. Express 19, 21475–21484 (2011)

Credit: Maurizio Burla

Reconfigurable optical delay lines (ODLs) and tunable, wideband phase shifters are important for microwave photonic applications such as signal filtering and arbitrary waveform generation. The common limitation of existing approaches is the trade-off between maximum achievable delay, operating frequency and bandwidth. Maurizio Burla and co-workers from The Netherlands have now presented a photonic signal processor that not only offers a wideband and fully tunable ODL, but is also monolithically integrated on a single compact CMOS-compatible chip. The processor consists of a reconfigurable ODL, a separate carrier tuning unit and an optical sideband filter. The optical sideband filter — a Mach–Zehnder interferometer loaded with an optical ring resonator in one of its arms — removes one of the radiofrequency sidebands of a double-sideband intensity-modulated optical carrier. The ODL and separate carrier tuning unit are individually implemented using a pair of cascaded optical ring resonators. Varying the group delay of the signal sideband by tuning the resonance frequencies and the coupling factor of the optical ring resonators in ODL, while also applying a full 0–2π carrier phase shift in separate carrier tuning, allowed the researchers to demonstrate a two-tap microwave photonic filter whose notch position can be shifted by 360° over a bandwidth of 1 GHz.